1. Field of the Invention
The present invention relates to a toner supply roller used in an image forming apparatus of a toner developing system such as a copying machine, a facsimile machine or a laser beam printer.
2. Description of the Related Art
An image forming apparatus of a toner developing system comprises in general a photosensitive drum, an electrostatic latent image forming means for forming an electrostatic latent image on the photosensitive drum, a developing roller rotated in direct contact with or close to the photosensitive drum, a toner supply roller for supplying toner to the developing roller, and a toner regulating member for regulating the toner supplied onto the developing roller to a uniform thickness. First, an electrostatic latent image is formed on the surface of the photosensitive drum by the electrostatic latent image forming means, based on prescribed picture image information. On the other hand, toner is supplied onto the surface of the developing roller by the toner supply roller. The toner supplied onto the developing roller is regulated by the toner regulating member to a uniform thickness so as to form a thin toner layer of a uniform thickness on the surface of the developing roller. The developing roller having the thin toner layer of a uniform thickness formed on the surface permits the toner to be attached successively to the electrostatic latent image formed on the photosensitive drum in the nip portion, or in the vicinity thereof, between the photosensitive drum and the developing roller, thereby achieving toner development.
The toner supply roller used in the image forming apparatus of the toner developing system described above comprises a metallic core shaft and an electrically conductive elastic layer formed on the outer surface of the shaft. In the prior art, the conductive elastic layer has been formed by dispersing an electrically conductive carbon used as an electrically conductive material into a resin foam such as a polyurethane foam by kneading. However, in the case of preparing a toner supply roller having a generally required surface resistance of 106 to 1010Ω by using an electrically conductive carbon, the conductive carbon fails to be dispersed uniformly into the resin material. As a result, the surface resistance is rendered nonuniform depending on the lot, depending on the roller in the same lot, or depending on the site of the same roller. Thus, it has been difficult to manufacture a toner supply roller having a nonuniformity in the surface resistance lower than 10%.
Therefore, it is an object of the present invention to provide a toner supply roller comprising an electrically conductive elastic layer exhibiting a substantially uniform surface resistance.